Steam power plant for generating electric power

ABSTRACT

A steam power plant for generating electric power has a fossil-fuelled boiler, a water-steam cycle for generating high-tension, superheated steam for a steam turbine, an economizer to transmit heat from flue gas to the feed-water, an air preheater to transmit flue gas heat to fresh air and devices for removing dust, sulphur and possibly nitrogen from the flue gases. In order to optimize heat exchange in the air preheater during operation and reduce the heat loses during start-up of the steam power plant, a heat exchanger system is provided with sections through which recirculated air and a heat vehicle medium flow, in which the section carrying the air is connected on the intake side to the fresh-air outlet of the air preheater and on the outlet side to the fresh-air intake of the air preheater.

BACKGROUND OF THE INVENTION

The invention describes a steam power plant for generating electricpower which has a fossil-fuelled boiler, a water-steam cycle forgenerating high-tension, superheated steam for a steam turbine, aneconomizer to transmit flue gas heat to the feed-water, an air preheaterto transmit flue gas heat to fresh air and devices for removing dust,sulphur and possibly nitrogen from the flue gases.

In steam power plants which are operated with fossil fuels, i.e.gaseous, fluid or solid fuels, the heat flux capacities (mass flow ratex specific heat capacity) of the flue gas which is to be cooled and thefresh air which is to be heated (combustion air) are different so thatthere is a temperature difference of up to 90° C. at the cold end of theheat exchanger with a customary temperature difference between flue gasand fresh air of approximately 30° C. at the warm end of the airpreheater. These high differences in temperature result in correspondingenergy losses and have a corresponding negative effect on the overallefficiency of the power plant.

Another disadvantage is that in power plants which are not equipped tofurther use the residual heat which is still at a relatively hightemperature level in the flue gas which leaves the air preheater, be itthrough reheating the cleaned flue gas before it enters the chimney orbe it through decoupling the heat for long-distance energy use, theresidual heat is destroyed in the flue gas desulphurizing plant. Theresult is another decrease in the power plant's overall efficiency.

Another factor is that the start-up process in the known steam powerplants described in the introduction is not advantageous. For example,before coal can be burnt in a coal-fuelled plant, considerable amountsof expensive auxiliary combustibles such as oil or gas must be burned inthe boiler before the coal can be burnt until the parts of theplant--for example the mills for the lignite pulverizer dryer of thecoal, the catalytic nitrogen removal reactor and the air preheater withits large regenerative heat accumulator masses, which must be heatedwith the help of the flue gas heat--reach their required minimumoperating temperatures. Furthermore, the steam which is produced duringthe start-up but also during the shut down phase generally isprecipitated in the capacitor of the steam power plant without utilizingthe heat.

The invention was charged with reducing the loss of energy in a powerplant as described in the introduction, to better utilize the heat ofthe flue gas and to make the start-up process more economical overall byusing less oil or gas and by better utilizing the steam which isgenerated during the start-up phase.

SUMMARY OF THE INVENTION

This task is solved in accordance with the invention through a steampower plant which is characterized in that it has a first heat exchangersystem with sections through which recirculated air and a heat vehiclemedium flow and in which the section carrying the air is connected onthe intake side to the fresh-air outlet of the air preheater and on theoutlet side to the fresh-air intake of the air preheater.

Compared to the state of the art, the measures proposed in accordancewith the invention result in a clear reduction of energy losses in theair preheater, in a considerably improved utilization of the heat whichis contained in the flue gas, and therefore in a marked increase of theoverall efficiency of such a steam power plant.

The recirculation air stream of the first heat exchanger system which issuperimposed to the fresh air in the area of the air preheater makes itpossible to almost completely align the heat flux capacities in the twoheat exchanger sections of the air preheater. The result is that thereare low temperature differences at the warm, as well as at the cold end,and correspondingly reduced energy losses. Some of the flue gas heatwhich is released in the air preheater which was transmitted torelatively cold fresh air in the customary design is now directlycoupled into the water-steam cycle of the power plant at a highertemperature level via the recirculation air stream and the first heatexchanger system.

Since the recirculation air stream in accordance with the inventionmakes it possible to regulate the equilibrium of the heat fluxcapacities between the two sections in the air preheater, the fresh aircan be preheated before it is introduced into the air preheater, i.e.the residue heat which is contained in the flue gas before it isdesulphurized can be used in the entire process due to the transmissionof the fresh air into the second heater exchange system in accordancewith the invention. The result therefore is a further improvement of theoverall efficiency of the power plant.

The invention is especially advantageous for power plants in which thedesulphurized cold flue gases are introduced directly into the coolingtower of the power plant and are released into the atmosphere togetherwith the cooling air and in which it is possible to decouple the residueheat of the flue gas for long-distance energy uses or other uses. Inthese cases the design in accordance with the invention makes itpossible to reintroduce the entire residual heat which is stillcontained in the flue gas into the power plant cycle underthermodynamically favorable conditions.

In power plants which use the residual flue gas heat either forlong-distance energy purposes or for heating the desulphurized fluegases which are to be released into the atmosphere via a chimney, theoverall efficiency can be improved with the help of anothercharacteristic of the invention by preheating the fresh air before itenters the air preheater, in a steam-air-preheater by transmitting thelow temperature heat from the water-steam cycle.

The design in accordance with the invention generally makes it possibleto bring the low temperature heat, which is produced in the power plant,to a higher temperature level by transmitting it to the fresh air, bycoupling it into the air preheater and by reintroducing it into thewater-steam cycle.

Important advantages with regard to the start-up phase of a steam powerplant can be achieved if--as intended by the invention--the first heatexchanger system is used as a start-up heat exchanger. This makes itpossible to preheat the regenerative heat accumulations of the airpreheater already before the start-up phase of the steam power plantbegins, i.e. already before the burners in the boilers are ignited.

For this purpose heat is transmitted to the recirculation air which isin the cycle between the still cold air preheater and the start-up heatexchanger via any heat vehicle in the start-up heat exchanger, and theregenerative heat accumulations of the air preheater are heated in theprocess.

Due to the preheating of the fact that the air preheater is preheated,the fresh combustion air, which is introduced into the boiler via theair preheater during the consequent start-up of the steam power plant,is heated correspondingly. The results are advantageous effects duringcold-start with boiler surfaces that are already cold, as well as duringhot start with boiler surfaces that are still hot. The plant heats upfaster during a cold start, i.e. less oil or gas is needed in the boilerwhile there is a minimal super-cooling due to inflowing cold combustionair during a hot start.

The advantage with coal-fuelled steam power plants--and this constitutesthe most important application of the invention--is that by preheatingthe air preheater, the minimum temperatures following the air preheater,which are required for the start-up of the coal heat lignite pulverizerdryer, can be reached quicker. The result is that it is possible toswitch faster from the oil or gas fuelling, which is customary duringthe start-up, to the normal coal fuelling. Another advantage is that theoperating temperatures needed for the nitrogen removal reactor can bereached faster while the efficiency of the nitrogen removal, andtherefore the effects on the environment, are influenced accordingly.

Another advantage which results from preheating the air preheater isthat when flue gas which is not yet desulphurized is first introducedinto the air preheater, a strong cooling with a corresponding dew pointis avoided which results in a correspondingly reduced corrosion insidethe air preheater, as well as in any subsequent parts of the facility,e.g. the electrostatic filter.

As already stated, the start-up heat, which is coupled into the airpreheater via the start-up heat exchanger, can be from any source. Itcan be, for example, the heat from another steam power plant which islocated at the same site, or it can be the waste heat of any otherindustrial plant.

For reasons of practicality, another characteristic of the invention isthat the heat vehicle is feed-water from a feed-water container of thewater-steam cycle, which is already heated to temperature by thestart-up steam from the steam power plant, from neighboring plants orfrom separate boilers.

If necessary, the temperature of the used feed-water can be furtherincreased in another heat exchanger through heat exchange with thecondensed start-up steam before it is cooled in the start-up heatexchanger.

Further explanations concerning the invention can be taken from theexamples which are shown schematically in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a circuit lay-out for decreasing heat lossesduring the operation of a steam power plant.

FIG. 2 shows an example of a circuit lay-out for decreasing heat lossesduring the start-up phase of a steam power plant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with FIG. 1, hot flue gas from a steam generator of acoal-fuelled power plant is transmitted to a nitrogen removal plant 2and then to an air preheater 3 with a temperature of approximately 380°C. via a line 1. Inside the air preheater 3 the flue gas is cooled to130° C. due to the heat exchange with air. After it has passed throughan electrostatic filter 4 and an induced draught ventilator 5, the fluegas is further cooled from approximately 130° C. to approximately80°-90° C. in a heat exchanger 6 of a second heat exchanger system whichconsists of heat exchangers 6 and 11 as well as a cycle water system 12.Then the cooled flue gas is fed into a flue gas desulphurization plant 8via a line 7 and then is released into the atmosphere together with thecooling air via the cooling tower of the power plant which is not shown.

The combustion air which is needed inside the steam generator issupplied to the power plant via a line 9 and a ventilator 10 and atfirst is preheated to a temperature of approximately 70-°80° C. in aheat exchanger 11. The heat needed for the preheating process istransmitted via a closed cycle water system 12 by the heat exchanger 6into the heat exchanger 11.

At a mixing point recirculating air--whose temperature and mass flux issuch that there is an approximate heat flux equilibrium inside the airpreheater 3, i.e. the desired small temperature differences between theflue gas and the air are now present at the cold as well as the warm endof the air preheater--is added to the fresh air which is preheated inthe heat exchanger 11.

After the recirculation air stream has passed the air preheater, it isagain separated from the fresh air stream at a separation point 14.While the fresh air is supplied to the fuelling of the steam generatorvia a line 15 and at a temperature of 350° C., the recirculation air iscooled again in a first heat exchanger system in a heat exchanger 16 byexchanging heat with high pressure feed-water and, if necessary, in asecond heat exchanger 17 by exchanging heat with low-pressure feed-waterand then is transported back to the mixing point via an adjustableventilator 18.

FIG. 2 shows schematic sections of a circuit of a coal-fuelled steampower plant. The hot flue gas is supplied from an economizer 21 of thesteam generator power plant via a line 22 to a catalytic nitrogenremoval reactor 23 and finally to an air preheater 24. Inside theeconomizer 21 the flue gas is cooled to the optimal operatingtemperature of the nitrogen removal reactor 23 of approximately350°-380° C. through heat exchange with feed-water. It is cooled toapproximately 130° C. through heat exchange with fresh combustion air inthe subsequent air preheater 24. After it is cooled down, dust orsulphur are removed from the flue gas in devices which are not shown,and then the flue gas is released into the atmosphere together withcooling air via a cooling tower which is also not shown.

The combustion air which the plant needs for the boiler is supplied viaa line 25, is heated to approximately 350° C. in the air preheater 24,and then is supplied to the fuelling or lignite pulverization dryer viaa line 26.

The shown section of the water-steam cycle of the plant shows afeed-water container 27 in which the condensate, which is supplied vialine 28, is heated by the steam from line 29. The heated water(feed-water) is removed from the feed-water container 27 via a line 30which is pumped up to approximately 250-300 bar in a jetting pump andthen is preheated to a temperature of approximately 250°-300° C. in acustomary high-pressure preheater 32. The preheated feed-water flowsinto the economizer 21 via a line 33 in which is heated again throughheat exchange with hot flue gas. The feed-water then is brought into theother heat exchanger system of the boiler via a line 34 and thereevaporates or is superheated to the starting temperature of the steamturbine of approximately 530°-580° C.

After the pressure in the turbine is relieved, the steam is condensedand is again brought to the feed-water container 27 via line 28.

The above description of the system of a steam power plant is based onnormal operations for full loads or partial loads.

The invention intends to make the start-up process of such a power plantmore economical. For this purpose the invention calls for a start-upheat exchanger 35 with a section through which recirculated air flowsand which is connected on the intake side to the fresh air outlet of theair preheater 24 via a line 36 and on the outlet side to the fresh airintake of the air preheater 24 via a line 37 and a ventilator 38.

Before or during the start-up process of the power plant therecirculated air which is between the air preheater 24 and the start-upheat exchanger 35 is heated inside the start-up heat exchanger 35 and iscooled again in the air preheater 24, whereby the regenerative heataccumulations of the air preheater heat up. This preheating process onone hand causes the flue gas, which is produced at the beginning of thestart-up process, to not be cooled as much inside the air preheater sothat the dew point underflow and related corrosion damage inside the airpreheater and any subsequent plant devices can be prevented.Furthermore, it is possible to transmit additional heat to thecombustion air via the recirculated air during the start-up process.This in turn makes it possible to reach the required temperature forstarting the lignite pulverization dryer and therefore the temperaturefor starting the coal burner of the boiler quicker. The result is thatnow the auxiliary burners, which are operated with expensive oil or gas,can be shut off earlier, and the flue gas side can reach the operatingtemperatures quicker (corrosion).

In accordance with the example shown in the figure, the recirculated airof the heat exchanger 35 is heated through heat exchange with hotfeed-water which is drawn off in the feed-water container 27 byinjecting start-up steam which is transported via a line 29 and vialines 39, 40, 41 with the correspondingly opened valves 42 and 43, iscooled in the heat exchanger 35, and then is returned back into thefeed-water container 27 via lines 44 and 45 and the open valve 46.

If necessary, the feed-water can be heated again before it enters theheat exchanger 35 by supplying at least a partial stream of the heatedfeed-water in line 39 via a now open valve 47 and a line 48 into anotherheat exchanger 49. The heat exchanger 49 is heated with the help of thecondensed start-up steam from a source 50 which then is supplied intothe feed-water container 27 via a line 51.

The feed-water stream, which is further heated in the heat exchanger 49,is first supplied into line 41 and then into the heat exchanger 35 via aline 52 and an open valve 53.

In accordance with another model of the invention, not only the airpreheater 24 but also the economizer 21 is supplied with additional heatduring the start-up process of the steam power plant. For this purposeat least a partial stream of the feed-water which was heated in the heatexchanger 49 is fed into line 33 via an open valve 54 and lines 44 and55 and there is mixed with the feed-water from the high pressurepreheater 32. This is shown in the example in the figure. Now it ispossible to use the start-up heat. Due to the increase in temperature ofthe feed-water, the flue gas inside the economizer does not cool as muchduring the start-up process so that the minimum temperature for startingthe subsequent nitrogen removal reactor 23 can be reached faster.

The heat transfer onto the storage accumulation of the air preheater 24or the additional heat transfer onto the combustion air or thefeed-water makes it possible to use the heat of the steam which isproduced during the start-up phase. Storing the steam in the airpreheater surfaces also makes it possible to use the steam which isproduced in the course of the start-up phase.

The proposed coupling of the additional heat into the economizer provedto be beneficial not only during the start-up process of the powerplant, but it can also be used for supporting the flue gas temperature,and therefore for maintaining the optimal operating temperature of thenitrogen removal reactor 23 during light load operation of the powerplant.

By varying the amount of the recirculation air via the ventilator 38 aswell as the transmitted heat amount inside the heat exchanger 35, theflue gas temperature after the air preheater 24 can be maintained at aconstant level in the entire load area; this means that even during fullload operation it is possible to obtain an optimally low flue gastemperature without having to accept any low temperatures and thereforecorrosion during partial load, for example.

We claim:
 1. A steam power plant for generating electric power has afossil-fuelled boiler, a water-steam cycle for generating high-tensionsuperheated steam for a steam turbine, an economizer to transmit heatfrom flue gas to the feed-water, an air preheater to transmit flue gasheat to fresh air, and devices for removing dust, sulphur and possiblynitrogen from the flue gas and is characterized in that it has a firstheat exchanger system (16, 17) with a section through which recirculatedair and a heat vehicle medium flow, whereby the section carrying the airis connected on the intake side (14) to the fresh-air outlet of the airpreheater (4) and on the outlet side (13) to the fresh-air intake of theair preheater (4).
 2. The steam power plant in accordance with claim 1is characterized in that it has a second heat exchanger system (6, 11,12) which transmits at least a part of the residual heat, which is stillcontained in the flue gas before the desulphurization process, to thefresh air.
 3. The steam power plant in accordance with claim 1 ischaracterized in that the recirculation air stream of the first heatexchanger system (16, 17) can be adjusted (18).
 4. The steam power plantin accordance with claim 1 is characterized in that the heat carrierfluid in the first heat exchanger system (16, 17) is feed-water from thewater-steam cycle.
 5. The steam power plant in accordance with claim 4is characterized in that the heat in the first heat exchanger system(16, 17) which is to be removed for the most part can be transmitted tohigh pressure feed-water (16) and the remainder to low pressurefeed-water (17) of the water-steam cycle.
 6. The steam power plant inaccordance with claim 2 is characterized in that the second heatexchanger system (6, 11, 12) is a closed system with water as a heatvehicle.
 7. The steam power plant in accordance with claim 1 ischaracterized in that the fresh air can be heated through heat exchangewith draw-off steam from the water-steam cycle of the power plant beforeit enters the air preheater.
 8. The steam power plant in accordance withclaim 1 is characterized in that the first heat exchanger system is astart-up heat exchanger (35), whereby the section which carries the airof the start-up heat exchanger (35) is connected on the intake side tothe fresh-air outlet of the air preheater (24) and on the outlet side(37) to the fresh-air intake of the air preheater (24).
 9. The steampower plant in accordance with claim 8 is characterized in that the heatvehicle medium inside the start-up heat exchanger (35) is hot water fromthe feed-water container (27) of the water-steam cycle of the steampower plant.
 10. The steam water plant in accordance with claim 9 ischaracterized in that the feed-water can be further heated in a heatexchanger (49) which is heated with condensed steam before it is cooledinside the start-up heat exchanger (35).
 11. The steam power plant inaccordance with claim 10 is characterized in that the cooled feed-waterand the condensed steam can be fed (25, 51) into the feed-watercontainer (27).
 12. The steam power plant in accordance with claim 8 ischaracterized in that at least a part of the heat vehicle medium for thestart-up heat exchanger (35) can be used for increasing the feed-watertemperature before economizer (21).
 13. The steam power plant inaccordance with claim 12 is characterized in that the heat vehiclemedium is hot feed-water, which can be mixed with the feed-water, whichis fed (33) into the economizer (21) after it is heated (49) againthrough heat exchange with condensed steam.